1.1 Technical Field
The present invention relates to films made of compositions based on polyamides and on polyolefins and more particularly to compositions comprising:
such that the impact strength (Dart Test) is greater than that of a film containing the same proportions of (A), (C) and (D) but not containing (B).
The compositions of the invention can also be characterized by a structure with a polyamide matrix, that is to say that the matrix retains many of the properties of polyamides.
1.2 Description of Related Art
The prior Japanese Patent JP 04-314,741 describes films composed of a mixture of (i) polyamide-6 and (ii) polyetheresteramide. The polyetheresteramide is a block polymer obtained by condensation of polytetramethylene glycol and polyamide blocks containing carboxylic acid ends. The polyamide blocks originate from polycaprolactam (PA-6) or from polydodecalactam (PA-12). The amount of polyether-esteramide is between 0.5% and 9% by weight of the mixture.
The property of this film is to withstand bending at 0.degree. C. according to Method 101 c of the MIL-B-131 F standard. The number of holes produced during 1,000 bending operations is measured.
If the amount of polyetheresteramide is too low, the number of perforations is significant; if the amount of polyetheresteramide is too high, the transparency and the mechanical properties of the film are reduced.
In this reference, the polyetheresteramide present is such that the proportion or ratio by weight of the polyamide blocks to the polyether blocks is in the range from 45/55 to 30/70.
It is useful for the polyamide films which are used in the food industry to exhibit good resistance to bending, in particular, at low temperatures for the packaging of deep-frozen food. However, it is also essential for packaging films for deep-frozen products (or products to be deep-frozen) to withstand perforation from the projecting parts or from the sharp edges of these products. It is also useful, during transportation of the deep-frozen product and its handling, for the projecting parts and the sharp edges not to cause perforation of the film.
This property is also desirable for the packaging of sterilized medical supplies. Indeed, perforation of the film would cause entry of air and thus contamination.